New tab placed after current tab when middle-clicking

Summary:
New tabs should be placed after the currently active tab when using
middle click.

Test Plan:
Open new tab from the places panel using middle click, verify that the
Open new tab from the folders panel using middle click, verify that the
tab is opened after the current tab
Open new tab by middle clicking on the Back button, verify that the tab
is opened after the current tab
Open new tab by middle clicking on the Forward button, verify that the tab
is opened after the current tab

FEATURE: 403690

Reviewers: #dolphin, ngraham, elvisangelaccio

Reviewed By: #dolphin, ngraham, elvisangelaccio

Subscribers: kfm-devel

Tags: #dolphin

Differential Revision: https://phabricator.kde.org/D19201